The present disclosure is directed to a process for treating a producing well and particularly one for treating a well where production has fallen over a period of time. The normal completion process involved in producing a well after drilling includes cementing casing in the well and then making perforations by means of shaped charges which perforate through the casing and any surrounding cement which holds the casing in place. The perforations penetrate into producing formations. Assuming that fluid production is obtained, the production fluids flow from the formation into the cased well. The production fluids are removed to the surface normally by installing a production tubing string in the cased well. For instance, the production tubing string typically measures 23/8 inches, or perhaps 27/8 inches. This defines an annular spaced around the production tubing string within the cased well borehole. The zone of fluid production is normally isolated with packers or plugs. A plug is normally placed in the casing just below the perforations. This enables a column of production fluid to accumulate above the plug. For production, there is also a plug or packer positioned above the perforations, and the production tubing string extends through this packer. Thus, the produced fluids from the formations are removed upwardly through the production tubing string usually by pumping or by gas lift apparatus.
Normally, after the passage of time, there is some loss of formation pressure in the localized formation region immediately adjacent to the well borehole near the perforations into the formation. This loss of production is occasioned also by a loss of fluid flow velocity. As the velocity decreases, the small cracks and fissures in the vicinity of the perforations may become clogged or plugged with silt, clay or other formation debris which is generally referred to as "fines". As well be understood, a higher pressure drive will tend to move the production fluids more rapidly through the perforations and into the cased well borehole. That however declines as sediment or fines in the produced fluid collects in the cracks and fissures connecting with the perforations. In other words, as the pressure drive decreases, it is decreased even further by sediment or fines in the flowing production fluids which falls out in the immediate region of the perforations. While this is a localized effect, it is nevertheless detrimental to production of fluids even where the formation or reservoir has ample fluids for production. The decline in production requires remedial treatment. There are multiple ways to treat such a well including further stimulation of the well by means of high pressure fracture, an injection of acid, etc. The present disclosure is directed to a particular well stimulation process which can materially ehance the production. Thus, it is a wireline conveyed tool which can be lowered on a wireline through the production tubing. It can be lowered to a location adjacent to the perforations without shutting in the well. Acoustic or sonic energy is generated by the tool and the acoustic vibrations are coupled from the tool through the incompressible liquids which make up the formation fluids to impinge on the formation to change the nature of the formation and to interact on both the formation materials including any fines or sediments which may settle in the perforations, in addition, the acoustic energy interacts with the formation fluids. This interaction serves to reduce formation fluid viscosity thereby enhancing volume fluid flow at a given pressure.
The materials which are found immediately outside the perforations are often multi-phase materials involving some, perhaps much water, and different weights of petroleum fluids, some of which might be natural gas and some which might be so heavy as to be tar like in nature. All of this material can be found in a producing sand formation which may be described generally as a supportive matrix with some given range of permeability to permit fluid flow from the formation into the well. The intersticial spaces in the formation normally provide sufficient connected pore space to enable fluid flow. The well fluid can carry fines which are sufficiently small that they can lodge or settle in the interstices of the formation and thereby tend to clog or plug the small connected pores of the formation. While there is no simple model to describe this, it is sufficient to note that there is a relatively complex interplay between the various solid, liquid and gas phase components as described above which can plug or impede flow in this region.
The present procedure contemplates the irradiation of the formation with acoustic energy at a selected power distribution and frequency. This irradiation agitates the fines so that they go back into fluidic suspension and can be removed with formation fluid flow. In addition, the irradiation appears to reduce the viscosity of the well fluids, thereby enabling enhanced fluid flow. Repetitive irradiation appears to clear or clean the pores or passage ways at and surrounding the perforations into the formations, thereby enhancing well fluid production.
The present apparatus is described very generally as a sonde which is adapted to be lowered in a well borehole on a wireline which supports the device at specified depths for remedial treatment of perforations and the immediate regions just beyond the perforations. The sonde houses one or more acoustic transducers which are operated at selected frequencies. Frequencies up to perhaps 20 or 30 kilohertz are generated to provide the appropriate irradiation. Acoustic power of a few hundred to a few thousand watts is delivered to the formation. The power density ranges anywhere from about 1 watt/cm.sup.2 to values less than this. This power level is sufficient to break free coagulated fines, and to also reduce the viscosity of the well fluid so that the fluid may flow more readily. In summary, well stimulation can be accomplished by selectively and repetitively irradiating the region of the perforations to obtain this improved production flow rate.